Electric heating element construction



March 19, 1940. w B RN 2,194,193

ELECTRIC HEATING ELEMENT CONSTRUCTTON Filed Sept. 19, 1936 W/ 77155555 I W IN VEN TOR.

Patented Mar. 19, 1940 UNITED STATES ELEC'IRIC IgfiEMENT CONSTRUC- William A. Barnes, Mansfield, Ohio Application September 19, 1936, Serial No. 101.643

-1 Claims. (01. 219-44) My invention relates to improvements in electric heating element constructions especially as adapted for use in electrically heated appliances for household use and more particularly to ele- 5 ments used in electric casseroles, cookers and roasters.

In the past it has been customary in making heating elements of this type to wind the element wire around the vertical side walls of the cooking 1o well over a thin sheetof asbestos. Such asbestos insulation is hygroscopic, contains foreign materials of poor insulating value, and combustible fillers and binders which smoke when the element is first heated, giving of! an acrid smoke l5 disagreeable to the user and deleterious to the air spaces between the asbestos and the cooking well. In other words, with the conventional construction square and rectangular cooking wells as shown in the drawings herewith were not pos- 30 sible because the asbestos would fall away from the wall of the cooking well on the flat sides, leaving air spaces to cause poor conductivity of heat to the cooking well and hot spots in the heating element. Tapered walls to the cooking 35 well were not practicable because the asbestos could not be economically cut to fit such walls and because the entire element could come loose from the cooking well by dropping downward. In using the usual heavy round element wire of 40 nickel chromium alloy having considerable stiif ness the wire expands as it quickly heats up coming loose from its supporting surface and even breaking thru the outer layer of cement used to bind it in place, coming in contact with the min- '45 eral wool heat insulation, which is disastrous to ness of mice by coating the cooking well with an I adhesive, preferably by spraying on an initial coat or wetting of adhesive and then applying to the adhesive coating a layer of flake mica,.thev

flakes of which are of the order of one eighth to 5 one half inch across and very thin in thickness.

' It is usually necessary with most adhesives to apply successive layers of adhesive and mica or to direct the spray of adhesive into the stream of mica. flake as it is blown into place. In this way it is possible to build .up any desired thickness of mica insulation without air spaces, voids, or bare spots. a

I prefer to apply the mica flake with a socalled flock gun such as is commercially used to I. blow fibrous or flake materials and to so direct the stream of blown mica that itis intercepted by amore forceful spray stream of adhesive ma terial directed to the surface to be covered so that the flakes are coated with adhesive in mid stream and are carried to and compacted upon the wall surface to be covered. In this way I secure the best interleaving and compacting of the mica. together with economy of adhesive.

It should be understood that altho I use the 88 term adhesive to describe the material used to hold the mica flakes to the supporting wall that due to the thinness and lightness of the mica ent adhesives and liquids can be used satisfactorily.

With many types of adhesives I find that heating the cooking well assembly after the micais in place causes the adhesive to soften and flow between the flakes of mica drawing them into a closer bond by capillary attraction. In this way I find that a better bond and denser layer of mice. can be produced by the use of less adhesive.

The adhesives that I have used are of many types. I propose to use shellac glyptal, phosphoric acid, boracic acid, sodium silicate, various natural and synthetic resins and animal and vegetable glues. These substances are used dissolved or suspended in suitable liquids. These liquids are evaporated from the finished heating element by basing or by the heat of the heating wires and the interior of the cooking well.

element the first few times it is used.

The mica is preferably a flake mica ground from scrap and small pieces of mica of good insulating value and free from mineral inclusions. Such mica may be obtained ready for application for from two to eight cents per pound,.therefore the economy and low material cost of such an insulating coating can well be understood.

Next over the mica insulating layer I wind the heating element wire, preferably in the shape of a ribbon, rather thin and crimped at intervals, this to give a springiness which takes up the expension of the heating element ribbon as it is heated up. Over the ribbon may be applied another layer of the ilake mica, altho this will not ordinarily be required. Such an added layer reduces the electrical insulation leakage between the heating element and the body of the appliance. This leakage becomes more pronounced with increase in the size of the appliance, increase in temperature and increase in the moisture in the air.

Over the assembled mica and wire element is applied a coating of vitreous cement which will harden with heat. I prefer a cement made with aluminum oxide grains of from to mesh mixed with a small percentage of clay, glass cullet, boracic acid or sodium silicate. Several satisfactory cements are available and on the market. I prefer to allow the cement to dry in the atmosphere or to hasten the drying by a low temperature baking and then to apply an electric potential'to the heating element to bake it to a suitable hardness altho this may be done in a baking oven. It is desirable to have the cement quite firm before the mineral wool insulation is applied around the outside of the element as the abrasion incidentto the application of the wool might otherwise damage the cement coating. Also the cement coating should extend over the edges of the'mica to anchor same to the cooking well.

In the drawing, Fig. 1 is a plan view of a. master using my heating element. Fig. 2 is a vertical section of a cooking well. Figs. 3 and 3a show two viewsof the heating element ribbon. Fig. 41s a perspective view, partly in section, of a cooking well with heating element in place.

Figure 1 shows an appliance, such as a roaster, inwhich my heating element construction is used. Due to my invention such appliances are no longer limited'to oval or round shapes with vertical walls, but can be built rectangular or square and still use a, heating element wound around the cooking well. Also as shown in Figure 2 my heating element can beapplied to a. cooking well having tapered sides. In Figure 3 I show a short piece of the element wire ribbon and illustrate how I propose to crimp same to give it the springiness desired to compensate for expansion when heated so that it will not have a tendency to separate from the mica or to push thru the cement coating, which difllculties are common with round element wires. The use of 'ribbon' presents a greater area of wire surface to the cooking well, resulting in lower wire temperatures 'and lower temperature gradient between element Also ,I may use an element wire such as nickel or iron, having a high temperature coeflicient of resistance and thus secure an inherently self-regulating element which prevents damage to the appliance or food by overheating. V

In Figure 4 I show a. section 01' my heating aiesnas element assembled in place. i shows the cooking well usually of aluminum or vitreous enameled steel. The mica flake insulating layer is shown at 2 and over it the heating element ribbon 3. The cement layer 4 is spread over the element assembly contacting the cooking well at the edges 5. At 6 I show a new method of constructing the cooking well edge so as to interlock with the outside body of the appliance as shown at i. This construction is shown and claimed in my copending application Serial Number 43,211 except that that application is drawn to round or oval constructions only. At t I show a clip attached to the body of the appliance and extending under the edge of the cooking well to prevent the body warping away from the cooking well by the pressure of the mineral wool insulation at 9. This clip will be needed only in appliances having flat sides such as the square or rectangular.

It will be apparent from the foregoing that I have invented a very practical, economical, efficient, long lived and novel heating element, free from many disadvantages heretofore inherent in this type of element and particularly adapted to modern manufacturing methods.

In the use of appliances which are heated by an electric heating element such as I have described the adhesive holding the mica. in place gradually burns out leaving the mica without a binder between the flakes. It should be understood that the use of the adhesive is mainly for the purpose of holding the mica in place until the heating element can be wound over it and the cement applied and hardened. The pressure exerted by the heating element serves to compact the mica, also the heat applied to bake the cement lowers the viscosity of the adhesive and serves to compact the mica. As the element is repeatedly heated in use the remaining adhesive is driven oil but as it is driven off the cement coating is hardened by the heat and prevents displacement of the mice. or element wires.

I reserve the right to use other materials than mica. flake and mixed with the mica such as asbestos fibres, magnesium oxide, clay and glass cullet-flnely ground. These materials may be used to bind the mica into a more compact mass and to hold it in place at high temperatures.

These and other variations may be utilized withoutdeparting from the spirit of my invention.

- I claim:

1. In an electrically heated appliance of the class described, ametallic wall separating the heating element enclosure from the space to be heated, an electrically insulating layer upon the said metallic! wall, retained thereupon by an electric heating element of resistance wire, the wire being crimped at intervals to give the said wire an added elasticity.

2. In an electrically heated appliance of the class described, a. metallic wall separating the heating element enclosure from the space to be heated, an electrically insulating layer upon the said metallic wall retained thereupon by an electric heating element of flattened resistance wire the flattened wire being crimped at intervals and applied flat side to the said insulating ayer.

3. In an electrically heated appliance of the class described, a. metallic wall separating the heating element enclosure tom the space to be heated, an electrically insulating-layer upon the said metallic wall comprising a. layer of mice. flakes compacted upon the-wall and retained thereupon by an electric heating element of flattened resistance wire, the flattened wire being crimped at intervals to give the said flattened wire an added elasticity.

4. In an electrically heated appliance of the class described, a metallic wall separating the heating element enclosure from the space to be heated, an electrically insulating'layer upon said metallic wall comprising a layer of mica flakes compacted upon the wall and retained thereupon by an electric heating element of flattened resistance wire, the flattened wire being crimped at intervals and applied flat side to the insulating layer.

5. In an electrically heated appliance of the class described, a wall separating the heating element enclosure from the space to be heated, an electrically insulating layer of micaceous laminae compacted upon the said wall, the planes of the lamina of the said insulating layer being substantially parallel to the surface of the said wall, said layer of micaceous laminae compacted upon the wall and retained thereupon by the capillary action of a liquid between the adjacent surfaces of the laminae and the laminae and the wall, an electric heating element supported by said layer 01' micaceous laminae, said heating element comprising resistance wire crimped at intervals to increase its elasticity, and a cementitious layer retaining said heating element and micaceous laminae to the said wall upon the drying out or the said liquid.

6. In an electrically heated appliance of the class described, a wall separating the heating element enclosure from the space to be heated, an adhesive upon the heating element side of said wall, a layer of micaceous laminae compacted upon the said adhesive, the planes of the lamina of said layer being substantially parallel to the surface of the said wall, a heating element of resistance wire upon said layer of micaceous laminae, said heating element comprising resistance wire crimped at intervals to increase its elasticity,-and a cementitious layer retaining said heating element and micaceous laminae to the said wall upon the destruction of the said adhesive by heat.

'7. In an electricallyheated appliance of the class described, an insulating layer of micaceous laminae, interleaved and compacted upon a supporting wall, temporary retaining means between the lamina of the interleaving laminae and the laminae and the supporting wall, additional retaining means upon said layer of micaceous laminae including an electric heating elementof corrugated resistance wire and a cementitious layer, said additional retaining means retaining said layer or micaceous laminae to the wall upon the destruction of said temporary retaining means.

WILLIAM A. BARNES. 

